Preventing or reducing scale in wet-process phosphoric acid production

ABSTRACT

Methods for preventing or reducing the formation of scale in a wet-process phosphoric acid production process by intermixing a water-soluble functional organic reagent with a phosphoric acid at one or more step of the phosphoric acid production process in an amount sufficient to prevent or reduce at least one species of scale are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority from U.S. ProvisionalApplication No. 61/245,713, filed Sep. 25, 2009 the content of which isincorporated herein by reference in its entirety.

This application is also related to, without benefit of priority, U.S.application Ser. No. 12,888,853, filed Sep. 23, 2010, and U.S.application Ser. No. 13/053,988, filed Mar. 22, 2011.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to formulated reagents and methods for preventingor reducing scale formation in and/or on production equipment at thedifferent stages of the phosphoric acid production process.

2. State of the Art

Although phosphoric acid can be prepared by three routes—the thermalprocess, the wet process, and the dry kiln process—the wet process isthe most commonly-used process in phosphoric acid production. In theprocess, calcium phosphate rocks, which contain mostly calciumphosphate, are cleaned in the wash plant and grinded in the Ball millbefore fed into a series of reactors for digestion with sulfuric acidalong with recycled phosphoric acid from the process. The digestiontemperature typically ranges from 40° C. to 80° C. After completing thereaction series, the process stream is washed with evaporator condensatewhile being forced through a filter.

After digestion, the reaction slurry is filtered to separate phosphoricacid from Gypsum (calcium sulfate). The filtered, crude phosphoric acidis then sent to Clarifiers and Evaporators for further purification andconcentration. The purified phosphoric acid is either sent out asMerchant Grade Acid (MGA) or continued to make 69% P₂O₅Super PhosphoricAcid (SPA). The Gypsum is washed and dried before being sold forcommercial uses. Some of the crude phosphoric acid is concentrated to44% (P₂O₅) before sent for Monoammonium Phosphate (MAP), DiammoniumPhosphate (DAP) and ammonium phosphate-sulfate (APS) production.

Due to the supersaturated nature of the acid and the impurities in thephosphate ores, the concentration steps with respect to P₂O₅ renderseveral side reactions, causing scale formation in and/or on theequipment at different stages of the phosphoric acid production. Forexample, fluorosilicate is one of the more common scale species found inphosphoric acid production. It can be depicted by the followingequations:

More than 12-15 other types of scaling species can be found throughoutthe phosphoric acid production process and they pose significantchallenges for the industry. Plants normally have to shut downproduction every few weeks to physically clean up the scale using highpressure water and/or mechanical means. The economic impact for thescale-related issues is substantial, and the industry is in need of amore efficient scale prevention technology than the existing physicalmeans of post scale formation removal.

Conceptually, there are two basic types of approaches scale removal fromthe phosphoric acid production process—namely, the physical method andthe chemical method. There are several options for the physical method.In addition to the previously mentioned mechanical and water washmethod, magnetic separation (Wang, Chuhua; Benson, Robert F.; Martin,Dean F. Enhanced solubility of sodium fluorosilicate scale by magnetictreatment, Florida Scientist (1998), 61(1), 17-25) and ultrasonicmethods (Pandey, A. D.; Mallick, K. K.; Pandey, P. C.; Varma, S.Prevention of scale deposition on heat exchanger surfaces by use of highintensity ultrasonic waves during concentration of wet processphosphoric acid, Fertiliser News (1983), 28(6), 45-8) have also beenused as part of the physical approach. Another approach still, isavailable by using physically smoothed piping in phosphoric acidproduction (See DE 3039187).

With regard to the chemical method, this approach is normally done byadding reagents to change the degree of supersaturation, either toinduce precipitation before filtration, or to prevent scale fromforming. This is the preferred approach because it requires a limitedamount of capital investment and does not alter the existing process inthe phosphoric acid plants. It also does not require a large amount ofreagent and is therefore considered both environmental, and to have aminimal impact downstream.

However, most of the existing work addressing the scale problem inphosphoric acid production plants is based on the work for addressingscale issues in water boiler system. Some examples for scale treatmentin boiler water are such as copolymers of acrylic acid and2-acrylamido-2-methylpropane sulfonic acid (AMPS) (EP0271035). Thesepolymers were reported to reduce the amount of silica gel adhering tothe wall of the testing bottles. Other systems such as polyamine,phosphonic acid and carboxylic acid based monomers and polymers havealso shown effectiveness in scale removal in boiler water system (See,e.g., GB2424876, JP2002263690, and EP0677485).

Because water boiler systems differ vastly from the wet-processphosphoric acid production environment, this system does not provide thebest model for use in the phosphoric acid production process. The waterboiler systems usually have mild condition with a pH in the range of 8to 9, and a low concentration of dissolved salts. The wet-processphosphoric acid production environment, by contrast, normally containsharsh conditions with a low pH and a high solid content. Additionally,the scale in phosphoric acid plants have much more complicatedcomponents—containing more than 15 known species, such as Na₂SiF₆,K₂SiF₆, CaSiF₆.2H₂O, CaF₂, MgF₂, CaSO₄.2H₂O (Gypsum), MgSiF₆.6H₂O,Mg_(0.8)Al_(1.5)F₆.XH₂O (wherein X is a variable integer), MgH₂P₆O₇,CaSO₄, Al(PO₃)₃, NaK₂AlF₆, Ca₃(AlF₆)₂.4H₂O, MgNaAlF₆.2H₂O,Ca₄SO₄AlSiF₁₃.10H₂O (see for example, A. William Frazier, James R. Lehr,and Ewell F. Dillard, Environmental Science 8. Technology, 11, 1007,1977). Moreover, different phosphoric acid plants experience differenttypes of scale, and even within one plant, the type of scale can differgreatly from one location of the process to the other. Due to thecomplexity of the scale forming issues, it is a great challenge todevelop scale inhibition reagents for use in preventing and/or reducingscale in phosphoric acid plants.

For example, in a typical 52% phosphoric acid production, a 60 torrvacuum is applied in a boiler and 85° C. phosphoric acid is circulatingand heated up by a heat exchanger at 130° C. During this evaporationprocess, some scale is formed either at the boiler or at the heatexchanger. However, the scale formed at the boiler can be different thanthat formed at the heat exchanger. The slow forming scale such as thatformed at the heat exchanger, contain mostly magnesiumfluoro-aluminates; while the fast forming scale such as that formed onpipes, contain mostly sodium or potassium fluorosilicate.

Yet, there is very little information in addressing the phosphoric acidplant scale issue in an industrial setting. Even for academic studies,the results are scattered. For example, several articles mentionreagents for fluorosilicate inhibition in phosphoric acid production.(See L. Yang, Zhang Y., Huang, Y. Chemical Industry and Engineering(China), (2002), V 19(1), 1). Chinese patent CN1762857 discloses thatmixtures of phosphonic acids, polymers, and pesticides can effectivelyreduce the scale formation in wet-process phosphoric acid production.U.S. Pat. No. 5,120,519 discloses that high molecular weightpolyacrylamide and polyacrylic acid can prevent scale from adhering onthe surface of the phosphate rock and phosphoric acid. However, the useof most of these chemicals is not new and has been applied in the watertreatment system for scale control, and the mechanism of these reagentsis based mostly on their dispersant effect.

Accordingly, the compositions and methods presently available forpreventing and/or reducing scale in the phosphoric acid productionprocess require further improvement. Compositions and formulations thateffectively prevent and/or reduce scale, thereby enabling the phosphoricacid production plant to run longer without shutting down to removescale would be a useful advance in the art and could find rapidacceptance in the industry.

SUMMARY OF THE INVENTION

It has now been discovered that certain formulated, water-solubleorganic reagents are useful for preventing and/or inhibiting multiplespecies of scale formation in and/or on production equipment at variousstages of the wet-process phosphoric acid production. Such formulatedreagents extend the production time for making phosphoric acid byreducing the frequency of the washing/shut down time to remove scale,thereby improving the overall productivity of the equipment and plant.

Accordingly, in one aspect, the invention provides methods forpreventing or reducing at least one species of scale in a wet-processphosphoric acid production process by adding at one or more steps of theproduction process a water-soluble, functional organic reagent chosenfrom one or more of: a phosphonic acid derivative; sulfonic acid or acorresponding derivative; a carboxylic acid derivative; a phosphatederivative; a co-polymer having a first component chosen from any of thepreceding reagents and a second component chosen from a suitablepolymer, in an amount sufficient to prevent or reduce the scale,provided that the phosphonic acid derivative is not chosen from:amino-tri(methylene phosphonic acid) (ATMPA);1-hydroxyethylidene-1,1-diphosphonic acid (HEDPA);diethylenetriamine-penta(methylene phosphonic acid) (DTPMP);ethylenediamine methylene phosphonic acid (EDTMP); hydroxylethanephosphonothyl acetic acid (HPAA); andphosphonobutane-1,2,4-tricarboxylic acid (PBTCA). In certainembodiments, the reagent can also be blended with various polymers,which are known to those skilled in the art to which the inventionpertains.

These and other objects, features and advantages of this invention willbecome apparent from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingExamples.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THE INVENTION

As summarized above, the present invention is based in part on the useof water-soluble functional organic reagents for use in preventing orreducing scale formed in and/or on the production equipment in thephosphoric acid production process.

Definitions

As employed above and throughout the disclosure, the following terms areprovided to assist the reader. Unless otherwise defined, all terms ofart, notations and other scientific or industrial terms or terminologyused herein are intended to have the meanings commonly understood bythose of skill in the chemical arts. In some cases, terms with commonlyunderstood meanings are defined herein for clarity and/or for readyreference, and the inclusion of such definitions herein should notnecessarily be construed to represent a substantial difference over thedefinition of the term as generally understood in the art unlessotherwise indicated. As used herein and in the appended claims, thesingular forms include plural referents unless the context clearlydictates otherwise.

Throughout this specification, the terms and substituents retain theirdefinitions. A comprehensive list of abbreviations utilized by organicchemists (i.e. persons of ordinary skill in the art) appears in thefirst issue of each volume of the Journal of Organic Chemistry. Thelist, which is typically presented in a table entitled “Standard List ofAbbreviations” is incorporated herein by reference.

As used herein the term “phosphonic acid derivative,” “sulfonic acidderivative,” and “carboxylic acid derivative” refer to compounds havinga functional phosphonic acid, sulfonic acid, or carboxylic acid group,respectively, in the compound. Where a phosphonic acid or sulfonic acidappear together with a carboxylic acid in the same compound, thecompound will be termed a phosphonic acid derivative or sulfonic acidderivative as the case may be. For example, sulfosuccinic acid isconsidered a sulfonic acid derivative for purposes of this application.Similarly, phosphonoacetic acid and2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) are consideredphosphonic acid derivatives for purposes of this application.

The term “copolymer” as used herein refers to a polymer composed of twoor more different components, wherein the components are linked randomlyor in repeating sequences, or in blocks, or as side chains off the mainchain. Accordingly, a phosphonic acid derivative copolymer, for example,refers to a copolymer having a phosphonic acid derivative component(i.e., a first component) linked randomly or in repeating sequence withone or more other component (i.e., a second component). Those ofordinary skill in the art will appreciate that there are a variety ofdifferent components (i,e., monomers, polymers, copolymers) known tothem that could be used as a second component to form a copolymeraccording to the invention. Sulfonic acid copolymer or sulfonic acidderivative copolymers, etc. are similarly formed.

As used herein, and as would be understood by the person of skill in theart, the recitation of “a reagent” is intended to include salts andsolvates of that reagent as well as any stereoisomeric form, or amixture of any such forms of that reagent in any ratio.

When the reagents of the present invention are basic, salts may beprepared from acceptable non-toxic acids including inorganic and organicacids. Suitable acid addition salts for the reagents of the presentinvention include acetic, benzenesulfonic (besylate), benzoic,camphorsulfonic, citric, ethenesulfonic, fumaric, gluconic, glutamic,hydrobromic, hydrochloric, isethionic, lactic, maleic, malic, mandelic,methanesulfonic, mucic, nitric, pamoic, pantothenic, phosphoric,succinic, sulfuric, tartaric acid, p-toluenesulfonic, and the like. Whenthe reagents contain an acidic side chain, suitable acceptable baseaddition salts for the reagents of the present invention includemetallic salts made from aluminum, calcium, lithium, magnesium,potassium, sodium and zinc or organic salts made from lysine,N,N′-dibenzylethylenediamine, diethanolamine, and ethylenediamine.

All numbers expressing quantities of ingredients, reaction conditions,and so forth used in the specification and claims are to be understoodas being modified in all instances by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe specification and attached claims are approximations that may varydepending upon the desired properties sought to be obtained by thepresent invention. Additionally, each numerical parameter should beconstrued in light of the number of significant digits and ordinaryrounding approaches.

Scale from the phosphoric acid production process forms on heatexchangers, evaporators, concentrators, and pipes during the repetitiveflashing/cooling/concentrating process of the phosphoric acid productionprocess. A scale inducing system was set up to mimic this process,whereby hot or cold water is fed through a stainless steel tube whilethe tube is submerged in hot phosphoric acid solution. The temperaturegradient and free flowing solid causes the formation of scale on theoutside of the tube. This system is the basis for the correspondingExamples provided herein, and it also uses a control flask underidentical environment for comparison to the sample flask.

Methods

In a first aspect, the invention provides methods for preventing orreducing at least one species of scale in a wet-process phosphoric acidproduction process, the method comprising: adding at one or more step ofthe phosphoric acid production process a water-soluble functionalorganic reagent chosen from one or more of: a phosphonic acidderivative; sulfonic acid or a corresponding derivative; a carboxylicacid derivative; a phosphate derivative; a co-polymer having a firstcomponent chosen from any of the preceding reagents and a secondcomponent chosen from any suitable polymer, in an amount sufficient toprevent or reduce the scale, provided that the phosphonic acidderivative is not chosen from: amino-tri(methylene phosphonic acid)(ATMPA); 1-hydroxyethylidene-1,1-diphosphonic acid (HEDPA);diethylenetriamine-penta(methylene phosphonic acid) (DTPMP);ethylenediamine methylene phosphonic acid (EDTMP); hydroxylethanephosphonothyl acetic acid (HPAA); andphosphonobutane-1,2,4-tricarboxylic acid (PBTCA).

In one embodiment, the species of scale prevented or inhibited fromforming during the phosphoric acid production process includes, but isnot limited to, one or more of: Si₂F₆; Na₂SiF₆; K₂SiF₆; CaSiF₆/2H₂O;CaF₂; MgF₂; CaSO₄/2 H₂O; MgSiF₆/6 H₂O; Mg_(0.8)Al_(1.5)F₆/X H₂O (whereinX is an integer ranging from 2 to 20); MgH₂P₆O₇; CaSO₄; Al(PO₃)₃;NaK₂AlF₆; Ca₃(AlF₆)₂/4 H₂O; MgNaAlF₆/2 H₂O; and Ca₄SO₄AlSiF₁₃/10 H₂O.

In some embodiments, the water-soluble functional organic reagents canbe added at any step of the phosphoric acid production process, whichsteps are well known to those skilled in the art. In certainembodiments, for example, the adding step occurs at one or more of themilling step; the digesting step; the filtering step; the clarifyingstep; and the condensation/evaporation step of the phosphoric acidproduction process. In one embodiment the adding step occurs after thedigesting step of the phosphoric acid production process. In anotherembodiment, the adding step occurs at the condensation/evaporation stepof the process.

The reagent(s) may be intermixed in various ways, e.g., in a singlestage, in multiple stages, sequentially, in reverse order,simultaneously, or in various combinations thereof. For example, in oneembodiment, the reagent is added to form a pre-mix, then intermixed withthe phosphoric acid. In another embodiment, the reagent is formed insitu by separately inter-mixing the components of the reagent with thephosphoric acid. Various modes of addition will be found to beeffective.

The reagents that comprise a liquid (such as water, oil and/or alcohol)may be formulated in various ways, e.g., the solid reagent may besuspended (e.g., colloidal suspension), dispersed and/or slurried in theliquid, and/or the reagent may be suspended, dispersed, slurried and/ordissolved in the liquid. In one embodiment, the reagent is addedseparately to the phosphoric acid solution. In another embodiment, thereagent is premixed and added together to the phosphoric acid solution.

In one embodiment, the concentration of the water-soluble functionalorganic reagent is from 10 to 1000 g per ton of phosphoric acid (e.g.,10 g/ton, 20 g/ton, 30 g/ton, 40 g/ton, 50 g/ton, 60 g/ton, 70 g/ton, 80g/ton, 90 g/ton, 100 g/ton, 110 g/ton, 120 g/ton, 130 g/ton, 140 g/ton,150 g/ton, 160 g/ton, 170 g/ton, 180 g/ton, 190 g/ton, 200 g/ton, 210g/ton, 220 g/ton, 230 g/ton, 240 g/ton, 250 g/ton, 260 g/ton, 270 g/ton,280 g/ton, 290 g/ton, 300 g/ton, 310 g/ton, 320 g/ton, 330 g/ton, 340g/ton, 350 g/ton, 360 g/ton, 370 g/ton, 380 g/ton, 390 g/ton, 400 g/ton,410 g/ton, 420 g/ton, 430 g/ton, 440 g/ton, 450 g/ton, 460 g/ton, 470g/ton, 480 g/ton, 490 g/ton, 500 g/ton, 510 g/ton, 520 g/ton, 530 g/ton,540 g/ton, 550 g/ton, 560 g/ton, 570 g/ton, 580 g/ton, 590 g/ton, 600g/ton, 610 g/ton, 620 g/ton, 630 g/ton, 640 g/ton, 650 g/ton, 660 g/ton,670 g/ton, 680 g/ton, 690 g/ton, 700 g/ton, 710 g/ton, 720 g/ton, 730g/ton, 740 g/ton, 750 g/ton, 760 g/ton, 770 g/ton, 780 g/ton, 790 g/ton,800 g/ton, 810 g/ton, 820 g/ton, 830 g/ton, 840 g/ton, 850 g/ton, 860g/ton, 870 g/ton, 880 g/ton, 890 g/ton, 900 g/ton, 910 g/ton, 920 g/ton,930 g/ton, 940 g/ton, 950 g/ton, 960 g/ton, 970 g/ton, 980 g/ton, 990g/ton, 1000 g/ton of phosphoric acid). In another embodiment, theconcentration of the water-soluble functional organic reagent is from 50to 300 g/ton of phosphoric acid. In a preferred embodiment, theconcentration of the water-soluble functional organic reagent is 100g/ton of phosphoric acid.

The treatment times may vary, depending in many cases on the nature ofthe scale formation rate and/or the species of the scale. For example,if the scale is formed within 30 minutes of the treatment, the overalltreatment time may be just one hour. If the scale is not formed within 4hours of the treatment, the overall treatment time may be over one day.One of ordinary skill in the art would be able to determine theapplicable treatment time through routine means.

In one embodiment, the scale formed in the phosphoric acid productionprocess is prevented or reduced from 10 to 180 days, depending on theamount and type of scale.

The pH of the phosphoric acid, although not adjusted, should not bealtered by a value of 1 after the addition of the reagent for treatment.The preferred pH of the phosphoric acid should be in the range of 1-5before starting the method of the invention. In case the pH of thephosphoric acid dropped below 1, it can be adjusted by sodium hydroxideor soda ash. In case the pH of the phosphoric acid rose above 5, it canbe adjusted by addition of sulfuric acid or phosphoric acid.

In one embodiment, the water-soluble functional organic reagent is aphosphonic acid derivative chosen from: phenylphosphonic acid;phosphonoacetic acid; hydroxyethylamino-di(methylene phosphonic acid)(HEMPA); and mixtures thereof.

In another embodiment, the water-soluble functional organic reagent is asulfonic acid derivative chosen from: sulfosuccinic acid;5-sulfosalicylic acid hydrate; 4-sulfophthalic acid;N-(1,1-dimethyl-2-hydroxyethyl)-3-amino-2-hydroxypropanesulfonic acid(AMPSO); 3-amino-4-hydroxybenzenesulfonic acid; 1-dodecanesulfonic acidsodium salt; 3-sulfopropyl acrylate potassium salt;4-hydroxybenzenesulfonic acid solution;4,5-dihydroxynaphthalene-2,7-disulfonic acid disodium salt;hydroquinonesulfonic acid potassium salt; and mixtures thereof.

In still another embodiment, the water-soluble functional organicreagent is a carboxylic acid derivative chosen from:3,4-dihydroxyhydrocinnamic acid; 3,4-dihydroxybenzoic acid; gallic acid;caffeic acid; tartaric acid; and mixtures thereof.

In a further embodiment, the water-soluble functional organic reagent isa co-polymer comprising a phosphonic acid derivative, a sulfonic acid orcorresponding derivative, a carboxylic acid derivative, or a phosphitederivative as a first component and a second component chosen from anysuitable polymer including, but not limited to: polyethyleneimine;acrylamide/acrylate copolymer (CYANAMER P-70®) (available from CytecIndustries Inc., Woodland Park, N.J.); allyl sulfonic acid/maleicanhydride copolymer (CYANAMER P-80®)(available from Cytec IndustriesInc., Woodland Park, N.J.); poly-dimethylamine ephichlorohydrinehtylenediamine (SUPERFLOC® C573), poly-diallyl dimethyl ammoniumchloride (SUPERFLOC® C587)(available from Cytec Industries Inc.,Woodland Park, N.J.); poly(4-styrenesulfonic acid);phosphinopolycarboxylic acid; acrylic acid/acrylate/sulfonateco-polymer; polyacrylic acid (PAA); sodium polyacrylate (PAAS);methoxyphenyl maleamic acid (MPMA); maleic anhydride acrylic acidpolymer (MA-AA); AA-MA-acrylamido-methyl-propane sulfonate polymer(AMPS) hypophosphorous acid quadripolymer; AA-AMPS multipolymer;AA-acrylate copolymer T-225; and acrylic acid-2-methyl propanesulfonicacid acrylic polymer; and mixtures thereof. In certain embodiments, anyof the reagents and/or co-polymers can be further blended with asuitable polymer such as those described herein.

Preferred reagents for use in the methods of the invention include, forexample, one or more of phosphonoacetic acid; tannic phosphite;hydroxyethylamino-di(methylene phosphonic acid) (HEMPA); sulfonic acid;sulfosuccinic acid; 5-sulfosalicyclic acid hydrate;N-(1,1-dimethyl-2-hydroxyethyl)-3-amino-2-hydroxypropanesulfonic acid(AMPSO); 3-sulfopropyl acrylate potassium salt; 1-dodecanesulfonic acidsodium salt; 4-hydroxybenzenesulfonic acid solution;4,5,-dihydroxynaphthalene-2,7-disulfonic acid disodium salt;3,4-dihydroxyphenylacetic acid; 3,4-dihydroxyhydrocinnamic acid;3,4-dihydroxybenzoic acid; tartaric acid;polyethyleneimine-epoxy-hydroxysuccinate; and mixtures thereof.

In certain embodiments, the method can further include one or more stepof flocculating the phosphoric acid with a flocculating agent; leachingthe phosphoric acid with a leaching agent; precipitating the phosphoricacid with a precipitation agent; and filtering the phosphoric acid.Typical agents for use with these additional steps are known to those ofordinary skill in the art.

Other Embodiments

1. A method for preventing or reducing at least one species of scale ina wet-process phosphoric acid production process, the method comprising:

adding at one or more steps of the phosphoric acid production process areagent chosen from one or more of:

-   -   i) a phosphonic acid derivative;    -   ii) sulfonic acid or a corresponding derivative;    -   iii) a carboxylic acid derivative;    -   iv) a phosphite derivative; and    -   v) a copolymer comprising a first component chosen from (i)-(iv)        and a second component;        in an amount sufficient to prevent or reduce said scale,        with the proviso that the phosphonic acid derivative is not        chosen from: amino-tri(methylene phosphonic acid) (ATMPA);        1-hydroxyethylidene-1,1-diphosphonic acid (HEDPA);        diethylenetriamine-penta(methylene phosphonic acid) (DTPMP);        ethylenediamine methylene phosphonic acid (EDTMP);        hydroxylethane phosphonothyl acetic acid (HPAA); and        phosphonobutane-1,2,4-tricarboxylic acid (PBTCA).

2. A method according to embodiment 1, wherein the species of phosphoricacid scale is chosen from one or more of: Si₂F₆; Na₂SiF₆; K₂SiF₆;CaSiF₆/2 H₂O; CaF₂; MgF₂; CaSO₄/2 H₂O; MgSiF₆/6 H₂O;Mg_(0.8)Al_(1.5)F₆/X H₂O; MgH₂P₆O₇; CaSO₄; Al(PO₃)₃; NaK₂AlF₆;Ca₃(AlF₆)₂/4 H₂O; MgNaAlF₆/2 H₂O; and Ca₄SO₄AlSiF₁₃/10 H₂O, wherein X isan integer ranging from 2 to 10.

3. A method according to any of the preceding embodiments, wherein theadding step occurs at one or more of: the milling step; the digestingstep; the filtering step; the clarifying step; and thecondensation/evaporation step of the phosphoric acid production process.

4. A method according to embodiment 3, wherein the adding step occursafter the digesting step of the phosphoric acid production process.

5. A method according to embodiment 3, wherein the adding step occurs atthe condensation/evaporation step of the phosphoric acid productionprocess.

6. A method according to any of the preceding embodiments, wherein theadding step is performed in a single stage, in multiple stages,sequentially, in reverse order, simultaneously, or in combinationsthereof.

7. A method according to any of the preceding embodiments, wherein thereagent is added directly to the phosphoric acid or premixed with asolvent chosen from: water; oil; alcohol; and mixtures thereof.

8. A method according to embodiment 7, wherein the reagent is premixedwith a solvent and provided as a colloidal suspension, a dispersion, aslurry, or dissolved in the solvent.

9. A method according to any of the preceding embodiments, wherein thereagent is a phosphonic acid derivative chosen from: phenylphosphonicacid; phosphonoacetic acid; hydroxyethylamino-di(methylene phosphonicacid) (HEMPA); and mixtures thereof.

10. A method according to any of embodiments 1 to 8, wherein the reagentis a sulfonic acid derivative chosen from: sulfosuccinic acid;5-sulfosalicylic acid hydrate; 4-sulfophthalic acid;N-(1,1-dimethyl-2-hydroxyethyl)-3-amino-2-hydroxypropanesulfonic acid(AMPSO); 3-amino-4-hydroxybenzenesulfonic acid; 1-dodecanesulfonic acidsodium salt; 3-sulfopropyl acrylate potassium salt;4-hydroxybenzenesulfonic acid solution;4,5-dihydroxynaphthalene-2,7-disulfonic acid disodium salt;hydroquinonesulfonic acid potassium salt; and mixtures thereof.

11. A method according to any of embodiments 1 to 8, wherein the reagentis a carboxylic acid chosen from: 3,4-dihydroxyphenylacetic acid;3,4-dihydroxyhydrocinnamic acid; 3,4-dihydroxybenzoic acid; gallic acid;caffeic acid; tartaric acid; and mixtures thereof.

12. A method according to any of embodiments 1 to 8, wherein the reagentis a co-polymer and wherein the second component is chosen from:polyethyleneimine-epoxy-hydroxysuccinate; CYANAMER P-70®; CYANAMERP-80®; poly(4-styrenesulfonic acid); phosphinopolycarboxylic acid;acrylic acid/acrylate/sulfonate; polyacrylic acid (PAA); sodiumpolyacrylate (PAAS); methoxyphenyl maleamic acid (MPMA); maleicanhydride acrylic acid polymer (MA-AA); AA-MA-acrylamido-methyl-propanesulfonate polymer (AMPS) hypophosphorous acid quadripolymer; AA-AMPSmultipolymer; AA-acrylate copolymer T-225; and acrylic acid-2-methylpropanesulfonic acid acrylic polymer; and mixtures thereof.

13. A method according to any of embodiments 1 to 12, wherein thereagent is chosen from: phosphonoacetic acid;hydroxyethylamino-di(methylene phosphonic acid) (HEMPA); tannicphosphite; sulfonic acid; sulfosuccinic acid; 5-sulfosalicylic acidhydrate;N-(1,1-dimethyl-2-hydroxyethyl)-3-amino-2-hydroxypropanesulfonic acid(AMPSO); 3-sulfopropyl acrylate potassium salt; 1-dodecanesulfonic acidsodium salt; 4-hydroxybenzenesulfonic acid solution;4,5-dihydroxynaphthalene-2,7-disulfonic acid disodium salt;3,4-dihydroxyphenylacetic acid; 3,4-dihydroxyhydrocinnamic acid;3,4-dihydroxybenzoic acid; tartaric acid;polyethyleneimine-epoxy-hydroxysuccinate; and mixtures thereof.

14. A method according to any of the preceding embodiments, wherein theconcentration of the reagent is from 10 to 1000 g per ton of phosphoricacid.

15. A method according to embodiment 14, wherein the concentration ofthe reagent is from 50 to 300 g per ton of phosphoric acid.

16. A method according to embodiment 15, wherein the concentration ofthe reagent is 100 g per ton of phosphoric acid.

17. A method according to any of the preceding embodiments, wherein thescale is prevented or reduced for a period of time from 10 to 180 days.

18. A method according to any of the preceding embodiments furthercomprising removing the scale-causing metal ions from the resultingphosphoric acid.

19. A method according to embodiment 18, wherein the removal step isperformed by flocculating the phosphoric acid with one or moreflocculating agent.

20. A method according to any of embodiments 18 to 19 further comprisingleaching the phosphoric acid with one or more leaching agent.

21. A method according to any of embodiments 18 to 20 further comprisingtreating the phosphoric acid with one or more precipitation agent.

22. A method according to any one of embodiments 18 to 21 furthercomprising filtering the phosphoric acid.

23. A method for preventing or reducing at least one species of scale ina wet-process phosphoric acid production process, the method comprising:

adding at one or more steps of the phosphoric acid production process areagent chosen from one or more of:

-   -   i) a phosphonic acid derivative chosen from phenylphosphonic        acid; phosphonoacetic acid; hydroxyethylamino-di(methylene        phosphonic acid)(HEMPA); and mixtures thereof;    -   ii) sulfonic acid or a derivative thereof;    -   iii) a carboxylic acid derivative;    -   iv) a phosphite derivative; and    -   v) a copolymer comprising a first component chosen from (i)-(iv)        and a second component;        in an amount sufficient to prevent or reduce said scale.

24. A method for preventing or reducing at least one species of scale ina wet-process phosphoric acid production process, the method comprising:

adding at one or more steps of the phosphoric acid production process areagent chosen from one or more of:

-   -   i) sulfonic acid or a corresponding derivative;    -   ii) a carboxylic acid derivative;    -   iii) a phosphite derivative; and    -   iv) a copolymer comprising a first compnent chosen from        (i)-(iii) and a second component;        in an amount sufficient to prevent or reduce said scale.

EXAMPLES

The following examples are provided to assist one skilled in the art tofurther understand embodiments of the present invention. These examplesare intended for illustration purposes and are not to be construed aslimiting the scope of the embodiments of the present invention or theclaims appended hereto.

Phosphoric acid solutions used for reagent testing are obtained fromphosphoric acid plants such as Agrium, Inc. Canada (Plant A); Prayon,Inc., Georgia (Plant P); and The Mosaic Company, Florida (Plant M) at28%, 42%, 52% or 69% P₂O₅. ICP and XRD analysis shows the crudephosphoric acids differ greatly in their metal components, and thissometimes leads to difficulty in forming scale within a reasonableperiod. Accordingly, the scale formation is sometimes induced withsalts. In some cases, 0.1% to 10% NaCl, KCl or MgCl₂ salts are added toinduce particular scale formation. These crude samples contained 28% and69% P₂O₅ from Plant A, 30% and 54% P₂O₅ from Plant P and 30% P₂O₅ fromPlant M. These samples are used as is or diluted to proper concentrationby adding water, or adjusted to more concentrated solution by adding 86%commercial grade phosphoric acid. In some cases, 0.1% to 3% NaCl, KCl orMgCl₂ salts are also added to induce particular scale formation duringtesting.

Scale was induced in the following manner:

Step 1: Acid preparation—In this step, crude phosphoric acid is obtainedfrom phosphoric acid plants and is treated properly (as is, diluting,concentrating or adding salt as scale initiator) before placing into thejacket beakers (60° C. to 80° C.) for 0.5 to 2 hours.

Step 2: Testing equipments set up and chemical addition—After thetreatment, proper dosages of the functional organic reagents are addedto the phosphoric acid and agitated using stir bar while being heated bywater circulator at 60° C. to 90° C. In the meantime, a 316 L stainlesssteel tube is placed in each beaker along with the cover and plastictubings for water inlet and outlet. Alternatively, a graphite tube or a904 L stainless steel tube can be used and the temperature for the tubecan be 110° C. to 130° C.

Step 3: Scale formation—If a functional organic reagent to prevent orreduce scale is used, it can be added just before the conditioning(generally the additive is used as a solution containing 1-10% of activereagent). This solution is put into the treated phosphoric acid in thejacketed beaker and is heated with agitation at 60° C. to 80° C. for 30minutes before the tube waster is turned on and kept at that temperaturefor 2-12 hours. Two to nine such tests (beakers) are done at one time.At the end of the test, the tube is thoroughly rinsed and dried in anoven (80° C.) for 1-2 hours.

Step 4: Weighing and analysis of the scale—Considerable scale isobserved to form on the steel tube. The weight gain of the steel tube isa measure of the amount of scaling. The weight of scale formed isexpressed as a percentage of the average weight that formed on theblanks (i.e., no reagent is used) that were part of the same set oftests. Similarly, the total amount of scale is also a measure ofantiscalant activity and this may be expressed as a percentage of thetotal weight that formed in the blank experiments that were part of thesame set of tests. The scale is also analyzed by ICP and XRD for metalion and component information.

This test method is preferred because other test methods collect boththe scales and the insolubles, although the insoluble may be freeflowing in the acid stream in the real plant and thus not contribute assignificantly to the scale growth. In this test, the scale is collectedon the outside surface of the stainless steel tubes. The tubes areweighed and compared to the tubes without reagent treatment to calculatethe scale changes. The reagents are usually prepared in deionized (“DI”)water for final of 3% concentration for testing. Unless it is statedotherwise, the concentration reagent in the testing solution is at amaximum of 2000 ppm.

Care must be taken to ensure all the parameters, such as but not limitedto, mixing rate, tube temperature, jacket temperature, tube surfacequality, tube volume submerged, stir bar size and acid quality, areclose to one another, so that the result of scale inhibition comparisonwith the control sample will be meaningful.

Example 1

Four jacketed-beakers are positioned and clamped on top of an aluminumtray filled with DI-water over the four corners of the hot plates. Thebeakers are connected in parallel in respect to the water flow from theheating circulator. Phosphoric acid (synthetic or crude plant acidsample at 28%) is mixed well before evenly dividing into 4 beakers(450-700 g). The beakers are mixed simultaneously by stir bars at thesame speed. The hot plate is turned on to heat the water bath to atemperature of about 90° C. After the mixing in each beaker isstabilized, the power of the heating circulator is started. Once thetemperature of the circulator reads about 50-60° C., reagents are thenadded to the individual beaker (usually to three of them with remainingone as control).

The four pre-weighed U-shape tubes with series connection to tap waterare then submerged into corresponding beaker. Once the circulator readsabout 75° C., the tap water is turned on to cool the U-shape tubes. Theend of the tap water temperature coming out of the last U-shape tube isabout 25° C. The mixing in each beaker is continued and carefullymonitored for occasional stops. All tap water and heating waterconnections are monitored frequently for possible leaking anddisconnection.

After a two hour treatment (or until there is visible scale formed onthe tubes), the heating for the jacket and cooling water for the tubesare turned off along with the stirring and heating for the hot plate.The tubes are disconnected and rinsed in a beaker with 500 ml DI waterto remove the residual phosphoric acid on the tubes. The tubes are thendried in an oven for 1 hour at 80° C. and cooled to room temperaturebefore they are weighed to find out scale weight on the tubes by thefollowing equation: Percent scale reduction (increase)=100×(Wt of scalew/reagent−Wt of scale w/o reagent)/(Wt of scale w/o reagent). ICPanalysis and XRD analysis is submitted when necessary.

After the scale study is complete, the beakers are removed with clampsattached and used acid solutions are poured into a waste container. Thebeakers are cleaned and returned to their original positions for thenext run. The stainless steel tubes are cleaned, oven dried, and weighedbefore reused for the next run.

Results for various functional organic reagents for preventing orreducing scale are shown in Tables A-D below:

TABLE A Test results using single-compound phosphonic acid derivativereagents Dosage, Scale change on Examples Reagents mg/l Tube % vs. blank1 Phenylphosphonic acid 100 −15 2 phosphonoacetic acid 100 −53

TABLE B Test results using single-compound sulfonic acid derivativereagents Dosage, Scale change on Examples Reagent mg/l Tube % vs. blank3 Sulfosuccinic acid 100 −92 4 3-Sulfopropyl acrylate 100 −78 potassiumsalt 5 1-Dodecanesulfonic acid 100 −72 sodium salt 64-Hydroxybenzenesulfonic 100 −66 acid solution 73-Amino-4-hydroxybenzene- 100 −27 sulfonic acid

TABLE C Test results using single-compound carboxylic acid derivativereagents Percent Scale Dosage, change on Tube Examples Reagent mg/l %vs. blank 8 3,4-Dihydroxyphenylacetic 100 −58 acid 93,4-Dihydroxybenzoic acid 100 −42 10 Gallic acid 100 −10 11 Caffeic acid100 −17

TABLE D Test results using blend or copolymers as reagents Percent Scaleinhibition Dosage, on Tube Examples Reagent mg/l % vs. blank 12Phosphinopolycarboxylic acid 100 −9 13 Acrylic acid/acrylate/sulfonate100 −17 copolymer 14 allyl sulfonic acid/maleic 100 −58 anhydridecopolymer (available as CYANAMER ® P-80 from Cytec Industries Inc.) 15poly(4-styrenesulfonic acid) 100 −2.5 16 Tartaric acid + allyl sulfonic100 −33 acid/maleic anhydride copolymer (available as CYANAMER ® P80from Cytec Industries Inc.) (1:1) 17 1-hydroxyethan-(1,1-di-phosphonic100 −60 acid)(HEDP) + polyMA-AA (1:1) 18 1-hydroxyethan-(1,1-diphosphoic100 −75 acid) (HEDP) + polyacrylic acid (1:1) 193,4-dihydroxyhydrocinnamic 100 −35 acid + allyl sulfonic acid/maleicanhydride copolymer (available as CYANAMER ® P80 from Cytec IndustriesInc.) (1:1) 20 Sodium di-2-ethylhexyl 100 −55 sulfosuccinate (availableas AeroDri ® 104 from Cytec Industries Inc. Woodland Park NJ) +polydimethylamine epichlorohydrin ethylenediamine (available asSUPERFLOC ® C573 from Cytec Industries Inc. Woodland Park NJ) (1:1) 21Tartaric acid + poly-diallyl 100 −33 dimethyl ammonium chloride(available as SUPERFLOC ® C587 from Cytec Industries Inc. Woodland ParkNJ) (1:1) 22 3,4-dihydroxyhydrocinnamic 100 −72 acid + poly-diallyldimethyl ammonium chloride (available as SUPERFLOC ® C587 from CytecIndustries Inc. Woodland Park NJ) (1:1) 23 4,5-dihydroxynaphthalene- 100−52 2,7-disulfonic acid sodium salt + acrylamide/acrylate co- polymer(available as CYANAMER ® P70 from Cytec Industries Inc. Woodland ParkNJ) (1:1) 24 Blend of 70% petroleum sulfonate, 100 −66 20%butoxypolypropylene glycol, and 10% water (available as Aero ® 865 fromCytec Industries Inc., Woodland Park NJ) + Tannic phosphite (1:1) 25Sodium di-2-ethylhexyl sul- 100 −29 fosuccinate (available as AeroDri ®104 from Cytec Industries Inc. Woodland Park NJ) + poly-diallyl ammoniumchloride (available as SUPER-FLOC ® C587 from Cytec Industries Inc.Woodland Park NJ) (1:1) 26 allyl sulfonic acid/maleic anhydride 100 −42copolymer (available as CYANAMER ® P-80 from Cytec Industries Inc.) +poly-dimethyl- amine epichlorohydrin ethylenedi- amine (available asSUPERFLOC ® C-573 from Cytec Industries Inc. Woodland Park NJ) (1:1)

Example 2

The testing condition is similar to that for Example 1, but thephosphoric acid concentration is increased to 52%. The test is performedwith 55° C. tube temperature and 80° C. acid temperature or with 35° C.tube temperature and 70° C. acid temperature in order to increase thetemperature difference to enhance scale formation. In all the tests, 240rpm to 300 rpm agitation, 1 kg of acid and 100 ppm (3 g of 3% solution)reagents is used. The duration of the tests is between 2 to 6 hours.

Results are provided in Table E below:

Table E Test results for various reagents at 52% P₂O₅ Percent Dosage,Scale Example Reagent mg/l inhibition 27 Tannic phosphite 100 −78% 28Sulfosuccinic acid 100 −94% 29 1-Dodecanesulfonic acid 100 −67% sodiumsalt with dopamine 30 3,4-dihydroxyhydrocinnamic 100 −88% acid 31hydroxypolyethylenimino 100 −67% succinate

A typical experimental process for hydroxypolyethylenimino succinate isas following: 2 g disodium cis-epoxysuccinate was synthesized from knownprocedure and was mixed with 8.3 g 50% polyethylenimine (PEI) and heated(either neat or in CH₃CN) and stirred for 4 hr. at 80° C. After cooling,the viscous liquid was treated with hexanes after which it wassolidified. It was filtered and dried and weighted. The solid wasdissolved in water for concentration adjustment. The reagent testingresults are summarized in Table F and reported as percent of scaleformed versus blank, where no reagent is used.

TABLE F Test results using hydroxypolyethylenimino succinate PercentScale Dosage, inhibition on Examples Reagent mg/l Tube % vs. blank 32hydroxypolyethylenimino 100 −97.5 succinate

Various patent and/or scientific literature references have beenreferred to throughout this application. The disclosures of thesepublications in their entireties are hereby incorporated by reference asif written herein to the extent that such disclosures are notinconsistent with the invention and for all jurisdictions in which suchincorporation by reference is permitted. In view of the abovedescription and the examples, one of ordinary skill in the art will beable to practice the disclosure as claimed without undueexperimentation.

Although the foregoing description has shown, described, and pointed outthe fundamental novel features of the present teachings, it will beunderstood that various omissions, substitutions, and changes in theform of the processes as illustrated, may be made by those skilled inthe art, without departing from the scope of the present teachings.Consequently, the scope of the present teachings should not be limitedto the foregoing discussion, but should be defined by the appendedclaims.

We claim:
 1. A method for preventing or reducing at least one species ofscale in a wet-process phosphoric acid production process, the methodcomprising: adding to at least one stage of the wet-process phosphoricacid production process a scale preventing or reducing amount of areagent selected from the group consisting of i) sulfonic acid or acompound having a sulfonic acid functional group selected from the groupconsisting of sulfosuccinic acid; 5-sulfosalicylic acid hydrate;4-sulfophthalic acid;N-(1,1-dimethyl-2-hydroxyethyl)-3-amino-2-hydroxypropanesulfonic acid(AMPSO); 3-amino-4-hydroxybenzenesulfonic acid; 3-sulfopropyl acrylatepotassium salt; 4-hydroxybenzenesulfonic acid solution;4,5-dihydroxynaphthalene-2,7-disulfonic acid disodium salt;hydroquinonesulfonic acid potassium salt; ii) a compound having acarboxylic acid functional group selected from the group consisting of3,4-dihydroxyphenylacetic acid; 3,4-dihydroxyhydrocinnamic acid;3,4-dihydroxybenzoic acid; gallic acid; caffeic acid; tartaric acid; andepoxysuccinic acid; iii)a compound having a phosphite functional group;iv) a copolymer comprising a first component chosen from (i)-(iii) andone or more second component that is a copolymer or polymer selectedfrom the group consisting of polyethyleneimine; acrylamide/acrylatecopolymer; allyl sulfonic acid/maleic anhydride copolymer;poly-dimethylamine epichlorohydrin ethylenediamine, poly-diallyldimethyl ammonium chloride; poly(4-styrenesulfonic acid); acrylicacid/acrylate/sulfonate co-polymer; polyacrylic acid (PAA); sodiumpolyacrylate (PARS); methoxyphenyl maleamic acid (MPMA); maleicanhydride acrylic acid polymer (MA-AA); AA-MA-acrylamido-methyl-propanesulfonate polymer (AMPS) hypophosphorous acid quadripolymer; AA-AMPSmultipolymer; AA-acrylate copolymer T-225; and acrylic acid-2-methylpropanesulfonic acid acrylic polymer; and v) a reagent blend comprisingat least two reagents selected from subgroups (i)-(iv).
 2. A methodaccording to claim 1, wherein the species of phosphoric acid scale isselected from the group consisting of Si₂F₆; Na₂SiF₆; K₂SiF₆; CaSiF₆+2H₂O; CaF₂; MgF₂; CaSO₄+2 H₂O; MgSiF₆+6 H₂O; Mg_(0.8)Al_(1.5)F₆ +X H₂O,wherein X is an integer ranging from 2 to 10; MgH₂P₆O₇; CaSO₄; Al(PO₃)₃;NaK₂AlF₆; Ca₃(AlF₆)₂+4 H₂O; MgNaAlF₆+2 H₂O; and Ca₄SO₄AlSiF₁₃+10 H₂O. 3.A method according to claim 1, wherein the adding step occurs at one ormore stages selected from the group consisting of the milling stage; thedigesting stage; the filtering stage; the clarifying stage; and thecondensation/evaporation stage of the phosphoric acid productionprocess.
 4. A method according to claim 1, wherein the adding step isperformed in a single stage, in multiple stages, sequentially, inreverse order, simultaneously, or in combinations thereof.
 5. A methodaccording to claim 1, wherein the reagent is selected from the groupconsisting of tannic phosphite; sulfonic acid; sulfosuccinic acid;5-sulfosalicylic acid hydrate;N-(1,1-dimethyl-2-hydroxyethyl)-3-amino-2-hydroxypropanesulfonic acid(AMPSO); 3-sulfopropyl acrylate potassium salt; 4-hydroxybenzenesulfonicacid solution; 4,5-dihydroxynaphthalene-2,7-disulfonic acid disodiumsalt; 3,4-dihydroxyphenylacetic acid; 3,4-dihydroxyhydrocinnamic acid;3,4-dihydroxybenzoic acid; tartaric acid; polyethyleneimine-epoxy-hydroxysuccinate; a 1:1 blend of 3,4-dihydroxyhydrocinnamic acidand poly-diallyl dimethyl ammonium chloride; and mixtures thereof.
 6. Amethod according to claim 5, wherein the reagent is sulfosuccinic acid.7. A method according to claim 5, wherein the reagent is 3-sulfopropyacrylate potassium salt.
 8. A method according to claim 5, wherein thereagent is a 1:1 blend of 3,4-dihydroxycinnamic acid and poly-diallyldimethyl ammonium chloride.
 9. A method according to claim 5, whereinthe reagent is tannic phosphite.
 10. A method according to claim 5,wherein the reagent is 3,4-dihydroxycinnamic acid.
 11. A methodaccording to claim 5, wherein the reagent is hydroxypolyethyleneiminosuccinate.
 12. A method according to claim 1, wherein the concentrationof the reagent is from 10 to 1000 g per ton of phosphoric acid.
 13. Amethod according to claim 12, wherein the concentration of the reagentis 100 g per ton of phosphoric acid.
 14. A method according to claim 1,wherein the scale is prevented or reduced for a period of time from 10to 180 days.